The invention relates to a method for manufacturing a metal-ceramic substrate in which a metal layer is applied to at least one side of a ceramic substrate using a direct bonding process.
Metal-ceramic substrates, in particular copper-ceramic substrates, are used increasingly as a base substrate or printed circuit board in power modules designed for higher operating voltages, e.g. for operating voltages of 600 V and higher. One of the requirements of such power modules is a sufficiently high partial discharge resistance. This requirement corresponds to the knowledge that partial discharges, which occur during operation of such a module over an extended period, cause electrically conductive paths in the isolating areas of the module, which can weaken the isolation and eventually also cause extreme voltage punctures, resulting in the failure of the respective module.
The requirement for the highest possible partial discharge resistance applies to the entire module, i.e. each individual component of the module must fulfill the requirement for the highest possible partial discharge resistance. Since the respective metal-ceramic substrate is an essential component of the respective module, this requirement also applies to this substrate, although partial discharges that occur only within the metal-ceramic substrate cause no damage to the isolating effect there. The reason for the requirement for each individual component to have the necessary partial discharge resistance is, for example, that it cannot be determined by measurements of the finished module which individual component of the module is responsible for partial discharges in the module.
The measurement of the partial discharge resistance is defined in standard IEC 1278. According to this measuring principle, the respective test piece is first subjected in a first measuring or test phase to an isolation voltage that is considerably higher than the operating voltage and then, in a second measuring or test phase, is first subjected to a reduced, preparatory measuring voltage and finally to the actual measuring or test voltage, at which the partial discharge is then measured. The preparatory test voltage is then above the maximum operating voltage of the respective module and the actual test voltage is below the maximum operating voltage of the module. The partial discharge may not exceed a value of 10 pico Coulomb (pC) in this measurement.
In the production of metal-ceramic substrates, a method is known for manufacturing the metallization required for strip conductors, connectors, etc. on a ceramic, e.g. on an aluminum-oxide ceramic, by means of the “direct bonding” process or for metallization made of copper by means of “DCB” (Direct Copper Bonding) technology, the metallization being formed from metal or copper sheets, the surfaces of which comprise a layer or a coat (hot-melt layer) resulting from a chemical bond between the metal and a reactive gas, preferably oxygen.
In this method, which is described for example in US-PS 37 44 120 and in DE-PS 23 19 854, this layer or coating (hot-melt layer) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that the layers can be bonded to each other by placing the foil on the ceramic and heating all layers, namely by melting the metal or copper essentially only in the area of the hot-melt layer or oxide layer.
The DCB process then comprises, for example, the following process steps:                oxidation of a copper foil so as to produce an even copper oxide layer;        placing the copper foil on the ceramic layer;        heating the composite to a process temperature between approx. 1025 and 1083° C., e.g. to approx. 1071° C.;        cooling to room temperature.        
One disadvantage of the DCB technology is that the process causes defective spots to occur between the respective metallization (copper) and the ceramic. Although these defective spots hardly affect the thermal properties of a metal-ceramic substrate manufactured using the DCB technology, since the bond, i.e. the surface area of the bond between the ceramic and the metal with no defective spots in relation to the total surface of the transition between the metal and the ceramic is generally greater than 90%, a certain problem results from the defective spots with respect to the partial discharge resistance.
The process-related disadvantages of the DCB technology can be eliminated for example by use of the active soldering process. The disadvantage of this process, however, is that it requires a relatively expensive solder and that a complex, multi-stage process is required for structuring the metal layers applied to the ceramic, in order to remove all of the electrically conductive material (including solder) between adjacent conductor strips, contact surfaces, etc., or that the metal layer forming the respective metallization must be structured before bonding to the ceramic layer or the ceramic substrate.
It is an object of the invention is to present a method that eliminates or at least reduces defective spots despite the use of the DCB process, so that a negative effect on the properties of the metal-ceramic substrate and in particular on the partial discharge resistance can no longer be determined.